Diisocyanates which are liquid at room temperature have numerous advantages over solid diisocyanates. The most commercially important diisocyanates which are solid at room temperature are 4,4'-diphenylmethane diisocyanate and 2,4'-diphenylmethane diisocyanate. Numerous patents have issued relating to the liquification of diphenylmethane diisocyanates ("MDI").
One common route to liquification of MDI is through carbodiimidization. Typical of this process are the processes described in U.S. Pat. Nos. 3,152,162, 3,384,643, 3,449,256, 3,640,966, 3,641,093, 3,701,796, 4,014,935, 4,088,665, 4,154,752, and 4,177,205.
Other less common techniques include reaction with N,N-di-(2-hydroxy-propyl)aniline (U.S. Pat. No. 3,394,165) and heating with N,N'-disubstituted thioureas (U.S. Pat. No. 3,674,828).
The most common technique used to liquify MDI is through reaction with various hydroxyl functional materials. Thus, the prior art has described several types of liquid isocyanates:
1) reaction products of MDI with poly-1,2-propylene ether glycols having molecular weights of from 134 to 700 (U.S. Pat. No. 3,644,457); PA1 2) reaction products of MDI with a mixture of alkylene glycols containing at least three carbon atoms (U.S. Pat. No. 3,883,571 and 4,229,347); PA1 3) mixtures of i) 30 to 80% by weight MDI, ii) 5 to 25% by weight MDI uretonimine-containing derivatives, iii) 20 to 50% by weight reaction products of MDI with diols or mixtures of diols having molecular weights of less than 175, and iv) 0 to 10% by weight polymethlyene poly(phenyl isocyanates) (U.S. Pat. No. 4,031,026); PA1 4) reaction products of polyoxyethylene glycols of molecular weight of from 200 to 600 and polymethlyene poly(phenyl isocyanates) containing from 65 to 85% by weight of MDI, with the balance being higher homologues (U.S. Pat. No. 4,055,548); PA1 5) reaction products of MDI with polyols having molecular weights of from 240 to 1500 and functionalities of from 2.7 to 3.3 (U.S. Pat. No. 4,102,833); PA1 6) reaction products of MDI (of various 2,4'-isomer contents) with propylene glycol or poly-1,2-propylene ether glycols (U.S. Pat. No. 4,118,411) or with polyoxyethylene glycois (U.S. Pat. No. 4,115,429); PA1 7) mixtures of i) 50 to 90% by weight reaction products of MDI and blends of 50 to 100% by weight of polyoxypropylene diols or triols having equivalent weights of from 750 to 3000 and 0 to 50% by weight polyoxyethylene diols or triols having equivalent weights of from 750 to 3000, and ii) 10 to 50% by weight of polymethylene poly(phenyl isocyanate) containing 30 to 65% of diphenylmethane diisocyanate (U.S. Pat. No. 4,261,8520); PA1 8) mixtures of i) reaction products of MDI and a polyol blend of a polyol having a molecular weight of at least 1000 and a polyol having a molecular weight of from 115 to 300 and ii) liquified MDI (U.S. Pat. No. 4,321,333); PA1 9) reaction products of MDI and an N-substituted ethanolamine (U.S. Pat. No. 4,332,742); PA1 10) reaction products of MDI containing from 10 to 20% by weight of the 2,4-isomer and a 5500 molecular weight polyether triol (U.S. Pat. No. 4,448,904); PA1 11) reaction products of MDI with a phenyl-substituted alkane diol (U.S. Pat. No. 4,490,300) or with a monoallyl ether of trimethylolpropane (U.S. Pat. No. 4,490,301); PA1 12) reaction products of MDI with a mixture of a monohydroxyl compound, a poly-1,2-propylene glycol and a low molecular weight triol (U.S. Pat. No. 4,490,302); PA1 13) reaction products of MDI of various isomer contents with 1,3-butane diol and propylene glycol (U.S. Pat. No. 4,539,156), with neopentyl glycol and propylene glycol (U.S. Pat. No. 4,539,157) and 1,3-butane diol and neopentyl glycol (U.S. Pat. No. 4,539,158); PA1 14) reaction products of MDI with amines and tripropylene glycol (U.S. Pat. No. 4,883,909); PA1 15) reaction products of MDI high molecular weight polyols and tripropylene glycol (U.S. Pat. Nos. 4,442,235 and 4,910,333); PA1 16) mixtures of i) reaction products of MDI and a polyol of equivalent weight of from 30 to 600 and ii) polymethlyene poly(phenyl isocyanates) (U.S. Pat. No. 5,240,635); and PA1 17) mixtures of i) reaction products of MDI and polyoxytetramethylene glycols of equivalent weight of from 250 to 1500, and ii) up to 30% by weight of a second isocyanate which can be the 2,4'-isomer or a polymethlyene poly(phenylisocyanate) (U.S. Pat. No. 5,246,977). PA1 i) no more than 2% by weight, and preferably no more than 0.25% by weight, of the 2,2'-isomer, PA1 ii) from 6 to 50% by weight, preferably from 10 to 35% by weight, and most preferably from 12 to 25% by weight, of the 2,4'-isomer, and PA1 iii) from 48 to 94% by weight, preferably from 65 to 90% by weight, and most preferably from 75 to 88% by weight, of the 4,4'-isomer, and
A variety of different molded polyurethane parts can be produced by the reaction injection molding ("RIM") process. This process involves filling a closed mold with highly reactive liquid starling components within a very short time, generally by using high output, high pressure dosing apparatus after the components have been mixed. The RIM process has become an important process for the production of external automotive body parts and other types of molded products. The RIM process involves the intimate mixing of a polyisocyanate component and an isocyanate-reactive component, followed by the injection of this mixture into a mold for subsequent rapid curing. The polyisocyanate component is typically based on a liquid polyisocyanate. The isocyanate-reactive component contains a high molecular weight isocyanate-reactive component, typically a polyol and/or an amine polyether, and usually contains a chain extender containing amino and/or hydroxyl groups. U.S. Pat. No. 4,218,543 describes a RIM process currently being commercially used on a large scale. U.S. Pat. Nos. 4,433,067, 4,444,910, 4,530,941, 4,774,263, and 4,774,264 describe reaction injection molding processes for the production of polyurethane(urea) elastomers. As used herein, the term "polyurethanes" includes polyurethanes, polyureas, and polyurethane polyureas.
Although the process described in the '543 patent has met with commercial success, the industry has searched for ways to reduce the density of the molded parts while at the same time maintaining the overall excellent physical properties of the parts.